My dear Borja Bueu, if you want to try engine condition without instruments I recommend the following:
1st you start the engine in minimum, you take a rag (like an ideal white handkerchief), you put it at the exhaust output, for about 2 minutes or less, when you remove it, you see what color it has if it is dark gravel or oil overflows. That indicates that there is oil leak x the rings or by the valve guides.
2 ° Another test is to remove the oil filling plug or take the oil to measure the oil with the engine in minimum, if this splash is too much and the engine has a change in the behavior (the revolutions are lowered or the engine is headed) that indicates that there are compression problems (worn rings or guides with game)-
Try to see how you go, luck I wish you the best.